The team of Rositsa Yakimova, professor emerita at Linköping University (LiU), has been focused on graphene fabrication by thermal decomposition of silicon carbide (SiC).
Yakimova and her team have pioneered a unique high temperature graphene process for graphene growth on SiC. Upon heating, the surface of SiC crystals undergoes decomposition due to sublimation, forming a vapour phase without passing a liquid phase. As determined by thermodynamics of the SiC binary system, silicon has the highest vapour pressure among all vapour species above SiC. Hence, Si leaves while carbon resides on the surface and re-bonds to form C layers of honeycomb-lattice symmetry possessing in-plane sp2 bonding. The method developed at LiU is derived from decades of experience in growing SiC crystals by sublimation. The availability of profound understanding of crystal growth mechanisms has been an excellent prerequisite for graphene growth recipes. In the commonly used low index crystal orientation, SiC substrates have two non-equivalent faces which are terminated either by Si or C atoms.
Combining the competence of graphene fabrication with that of processing (Chalmers, Sweden) and quantum metrology (NPL, UK) has empowered, for the first time, the demonstration of a Quantum Hall resistance quantisation accuracy of three parts per billion in monolayer epitaxial graphene at 300mK, four orders of magnitude better than previously reported (Nature Nanotechn., 2010). In 2010, Professors Sirs Konstantin Novoselov and Andre Geim were awarded the Nobel Prize in Physics for isolating the 2D crystal graphene and demonstrating its non-classical physical properties. The same year one of the first EU projects on graphene, ConceptGraphene, aiming at wafer scale electronics, was funded, in which Yakimova supplied the graphene required. In the following years, graphene research has reached unbelievable hype. Yakimova has been involved in graphene-related projects funded by the EUROGRAPHENE programme, FP7 and EMRP Grant Researcher. She is a principal investigator (PI) in work package materials in the Graphene Flagship and a grant holder or PI of several national grants. The area of the graphene on SiC can be as large as that of the substrate. SiC wafers six inches in size are available on the market today and eight-inch wafers are under development. Most importantly, graphene continuity is not broken up by grain boundaries or other extended defects. Specific features of this type of graphene are the step edges which are intrinsically formed due to step bunching in SiC hexagonal polytypes (6H and 4H). An undesirable consequence of this phenomenon is the enhanced Si sublimation at the step edges which facilitates local bilayer graphene formation. The presence of bilayers can affect the sheet resistance of graphene and deteriorate device performance. This is more pronounced and therefore crucial when growing graphene on a wafer scale. The team led by Yakimova has reached sufficient understanding of the operation conditions through comprehensive analysis of the step bunching process and its control during high temperature graphene growth on SiC (Carbon, 2013).
Due to the increasing demand for large area electronic-grade graphene, the inventors of “high temperature graphene process” for graphene growth started up the company Graphensic AB as a spin off from LiU in 2011. The co-founders, Professor Rositsa Yakimova, Dr Mikael Syvajarvi and Dr Tihomir Iakimov have the mission to progress the company to become a leading supplier of wafer scale graphene not only in Europe but worldwide. The strong interaction between the company and the research group is a motor of the new product development. The company has acquired a graphene growth reactor with a capacity of three wafers up to four inches each. This is in line with the requirements of the Graphene Flagship for attaining industrial scale production of graphene. This will be facilitated by an immediate feedback from the research work package. Graphensic is a member of the executive board of the new Swedish initiative for graphene development (SIO Grafen). The aim is to strengthen graphene activities and collaborations on a national level, and drive the Swedish industry towards the production of graphene-based materials and systems.